We also stopped at parks in small towns and let them play on the playgrounds. Ice cream shops or child-friendly attractions were also a good diversion. Remembering that the kids are still kids, even if we're forming them into bicyclists, is helpful.
I can't think of a better education than crossing the nation on a bicycle. You're doing a good thing for your kids. You can always use the education and family-time aspects to help convince your wife. At their ages they will have already studied a lot of history, and it will be exciting for them to see these places for themselves, and at the pace of a bicycle, no less! The experiences and adventures they'll have are priceless.
We're starting over with a 16-month old grandson, now that are kids are grown and gone. We've already had him out in a baby-seat on the back of my bike, and will use the trailer on longer rides. He LOVES being on the bicycle. I hope it's the start of a long love relationship with him and the bicycle!